When a fork lift truck is driven in other than a straight line, the wheels nearer the centre of turning travel a smaller distance than the outer wheels.
EP 0 303 413 shows a narrow aisle lift truck having a body and a mast structure which are pivoted together about a main vertical or king pin axis. The body has wheels on a first common axis and the mast structure has feet with wheels on a second common axis. When the axes are parallel the parts are pivotally related for straight ahead, or straight back, travel. The mast has a carriage for elevation of forks which are for insertion and retraction of a load to and from the stack in which the aisle extends. Steering is effected by driving the mast structure about the king pin by any suitable means. A truck having all of these mentioned features is herein called "of the kind referred to".
A truck of this kind may, in an extreme position, have the said axes at an angle of the order of 90 deg. to one another. This can result in skidding and tire wear.
DE 1049307B shows a truck having front wheels only driven by separate motors which can be controlled individually as to direction of speed. This provides a solution to the problem of the inner and outer wheels moving differently, but because those motors act as the only steering means, there are other problems introduced.
DE 1209450B uses two hydrostatic motors, one for each driven wheel, which can run at different speeds when the truck is in curvilinear motion, but this only goes some way towards solving the problem and does not cater for the possibilities of extreme movement of the steering such as those mentioned in the foregoing paragraphs.
GB 2 167 024 shows a vibratory trench roller with four rolls, which can be controlled by so-called tank steering in which two rolls on one side are driven in the same or the opposite direction as those on the other side. However, this does not give satisfactory steering possibilities for a wheeled forklift truck.
The object of the invention is to provide improvements.